1. Field of the Invention
The invention relates to methods and thermodes for thermocompression bonding and, more particularly, to methods using a thermode enclosing a fusible material adjacent the tip.
2. Description of the Prior Art
The fundamentals of thermocompression bonding are set forth on pages 604- 612 of the book, R. W. Berry, P. M. Hall and M. T. Harris, Thin Film Technology, D. Van Nostrand, Princeton, N.J., Ch. 12 (1968). Briefly, thermocompression bonding is a solid-phase bonding technique which forms the bond between two members by inducing a suitable amount of material flow in one or both members. The material flow is induced by the application of heat and pressure, which are maintained for a suitable length of time so that adhesion takes place without the presence of a liquid phase. An anvil is used to support the members and a thermode is employed to press the mating surfaces together and deliver the necessary thermal energy for the time required to bond them. Typically, the temperature of the members initially rises very rapidly when the thermode contacts one of the members, and then rises gradually until contact is broken. If the gradient following the initial rise in temperature can be increased, the interface temperature between the mating surfaces will be increased and the time required to complete the bond can be shortened. This shortening, even in terms of a fraction of a second, is significant and important because of the very large number of bonds to be made. In terms of percentages, for example, one half of a second saved could mean a 25% to 50% reduction in bonding time.
The thermode must operate at about 800.degree.C which is a dull red heat. At this temperature most metals oxidize very rapidly and, therefore, the thermode is made of an oxidation resistant alloy such as stainless steel or similar material. These metals are poor thermal conductors compared to steel and copper which have thermal coefficients of conductivity roughly about four times as great and 20 times as great, respectively. As a consequence, heat lost from the tip of a stainless steel thermode or the like is slow to be replaced and the longer the heat path the slower it becomes. This aggravates the initial temperature drop and subsequent gradient and makes them worse than they would otherwise be if a good conductivity metal could be used.
The problem is further aggravated by the need to make the tip narrow to meet mechanical requirements. This precludes the possibility of positioning the primary source of heat for the thermode in close proximity to the tip.